The ability of painted exterior surfaces to remain relatively clean in appearance for long periods of exposure is a major performance issue in architectural coatings. Dirt has always caused problems for light-colored coil coated panels when they are used outside. This is especially important in areas of high pollution, where panels that initially start perfectly clean are soon transformed into dull and grimy surfaces by the action of dust or dirt. The dust and dirt are not removed by rainwater. It is reported that a building's appearance can be severely affected by adverse environmental conditions within just two months. Dirt is generally deposited on the surface of the coating as rainwater runs down it. The water drop falls away, leaving dirt, watermarks and unsightly streaks behind.
It has been shown that a hydrophilic surface may produce good dirt shedding. The hydrophilic coating resists dirt pickup since water can wet the surface and flow off more easily allowing dirt to be easily washed off by rainwater. This process is known as “self cleaning”.
Coatings containing silicates have been shown to provide superior stain resistance. Such coatings are described, for example in EP0942052, JP2002294154, and JP01172389. Organosilicates have also been used with fluoropolymer resins to produce desirable coating compositions as described in JP02003775, JP2003020450, U.S. Pat. No. 6,635,341 and EP1035184. While the addition of silicate additives leads to good dirt and stain resistance, they have the problem of producing a paint having a short pot life.
An organosilicate works by a mechanism of stratification first, then hydrolysis with rain water in the surface. When the silicate additive is formulated in paint, the silicate will migrate to the surface of the coating along with solvent evaporation during baking.
When it rains, alkoxy-silyl groups at the surface are hydrolyzed and form silanol groups, increasing the hydrophilicity. Both the migration and hydrolysis rates are crucial to the better dirt shedding performance. Since the organosilicate additives are hydrolytically sensitive materials, the hydrolysis and the condensation cross-linking reaction occurring in the paints will prevent silicates from migrating into the surface during baking, hindering generation of a more hydrolytic surface. If the hydrolysis rate is too low, a hydrophilic surface is hard to obtain. If it is too fast, the paint pot life will be very short.
Presently, the pot life of a paint or coating is only 1–2 days after an organosilicate is added into the formulations. Anything not used the same day has to be thrown out making the process very expensive. In addition, the initial gloss of a coated panel will be negatively affected by the addition of the silicate. Also, cracking problems could arise due to the cross-linking reaction between the additives.
Surprisingly it has been found that the addition of selected organo-silicate additives to a fluoropolymer coating composition, along with at least one water-scavenger, results in prolonged coating pot life. The hydrolysis and condensation reaction of the organosilicate additives are controlled to produce optimum properties. The coating composition forms a coating having superior weatherability and good dirt shedding performance, without a negative effect on gloss or other coating properties.